All you need to do is change one thing. Legionniares 3 never happened. We never had Cosmic Boy go back through time. We never had him become Night Girl's husband. So there was never a honeymoon. There was never a reason for Cosmic boy to go to the past. We never get the reveal that the Time Trapper shunted Legionniares into the pocket universe.

That's fine. If you remove the whole Pocket-Universe storyline and its effects and insert the revised Kal-el/Superboy as per the new Infinite Crisis continuity then in effect Legion continuity remains intact until at least the Crisis. However just because Superboy breaks contact with the Legion come the Crisis who are you to say that the established continuity and adventures that make up the latter half of the Levitz/LaRoque series also stops there? That the Legion never faced the new Starfinger, Foiled The Universo project, or fought The Magic Wars?
The way I interpreted the Geoff Johns revision and Paul Levitz' work on the modern series is that the established Legion canon is more or less intact up until the end of the late 1980s' sixty-three issue long series and what comes after The Magic Wars allows for the rise of Earth-Man and his exploitation of an underbelly of xenophobia on earth. As with his revision of Barry Allen's origin that racist allegory from Johns had all the subtlety and finesse of a brick,but if you accept that the earth had been caught in a moment of vulnerability and fear thanks to the Magic Wars then it makes a sort of plausible sense that that trauma could be easily bent into fear of other races and anything from beyond...

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Cosmic boy says that he sacrificed himself so that Garth and Imra could have a chance at happiness. Initially, in Legionniares 3, the Trapper targetted Garth as the "weak link." What if he targeted Rokk instead?

By your own observation the Time Trapper's Pocket-Univers didn't happen in the revised history, but I have no difficulty in accepting Legionnaires 3 did happen as it was hardly central to that later plot. Cosmic Boy referring to sacrifice is something you take too literal I feel, his reference is purely to the fact that he took the reigns of leading the group and acting as full-time figurehead so that Garth and Imra could be together and not have that pressure put on either or both.

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All the Trapper needed to do is stop his relationship with Night Girl. Maybe that's what is given up to save Garth and Imra's child. He gave up his love with Night Girl. This results in the Legion being blind to the Machinations of the Trapper.

I am sure that while their marriage was under severe strain come Paul Levitz' 2011 series the two were still nominally married. I seem to remember as much being the case.

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